Along with a recent increase in density and speed of semiconductor integrated circuits, the pattern line width of the integrated circuits is decreasing, and higher performance has been demanded for semiconductor manufacturing methods. Accordingly, for exposure apparatuses used for forming a resist pattern in lithography in the semiconductor manufacturing step, there have also been developed steppers using extreme ultraviolet light such as KrF laser (248 nm), ArF laser (193 nm), and F2 laser (157 nm), and exposure light such as X-rays (0.2 to 1.5 nm) having a wavelength shorter than a conventional one.
In exposure using X-rays, a proximity exposure method of moving an X-ray mask having a desired pattern to be close to a resist-coated wafer, and irradiating the wafer with X-rays through the X-ray mask, thereby transferring the projected image of the mask pattern onto the wafer, has been developed.
In order to obtain high-intensity X-rays, an exposure method using synchrotron radiation is proposed. The technique has been reported that a pattern of 100 nm or less can be transferred. A synchrotron radiation source requires large-scale facilities. A profit cannot be expected unless device fabrication is performed by connecting ten or more exposure apparatuses to one light source. Hence, an exposure apparatus using a synchrotron radiation source is a system that is suitable for application to a highly demanded device such as a semiconductor memory.
In recent years, a device using GaAs has been input into practical use as a communication device, and a large decrease in line width is required. Communication devices are produced in an amount less than that of semiconductor memories, and many types of communication devices are produced in small amounts. When an X-ray exposure system using synchrotron radiation as the light source is introduced to the fabrication of communication devices, it will probably make no profit. Hence, the exposure apparatus which is suitable for application to produce devices (not limited to the communication devices) in small amounts, and can transfer a micropattern at low cost is expected to be developed. To meet this demand, an exposure apparatus using a compact X-ray source which generates high-intensity X-rays is developed and used in actual communication device production. The light source ranges from one which is called a laser plasma beam source and generates a plasma by irradiating a target with a laser beam and uses X-rays generated by the plasma, to one which generates X-rays by generating a pinch plasma in a gas. These light sources are called point sources. According to a general arrangement, one exposure apparatus which transfers a pattern by aligning a mask and wafer is connected to one point source.
The proximity X-ray exposure is different from reduction projection exposure for reducing the size of the image of a transfer master (mask), and forming a reduced image on a target substrate (wafer) by using an optical system. The proximity X-ray exposure is a method of irradiating the wafer with the exposure light in the state of holding a transfer master (mask) and target substrate (wafer) via a small gap in parallel to transfer the pattern. The circuit pattern on the mask is transferred at x1. Since the alignment accuracy of the mask directly becomes the overlay accuracy of the circuit pattern, high-precision mask alignment is required. Hence, in the proximity X-ray exposure, the mask is aligned to a wafer stage coordinate system.
A conventional mask alignment system is disclosed in Japanese Patent No. 2,829,642, and the arrangement of the apparatus is shown in FIG. 7. A main body frame 51 includes a mask alignment scope 11 which executes an alignment measurement, a mask stage 41 which aligns a mask 20 in a rotational direction, and a wafer stage 31 which aligns the position and posture of a wafer 30, and moves step by step for each exposure processing. The mask alignment scopes 11 are mounted on respective 2-axis stages 501, and can move in X and Y directions. Reference numeral 33 denotes a laser interferometer beam for precisely controlling the position of the wafer stage 31. Reference numeral 502 denotes alignment light; and 503, reference mark table on which reference marks of the wafer stage are formed.
FIG. 8 shows an arrangement of the alignment portion in which four mask alignment scopes 11a, 11b, 11c, and 11d are arranged. A relative distance between each of mask-side alignment marks 504 formed on the mask 20 and each of wafer-side alignment marks 505 formed on the wafer 30 is measured by using alignment light 502. In this arrangement, the mask alignment scopes denoted by reference symbols a and b measure positions in the X direction, and that denoted by reference symbols c and d measure positions in the Y direction. On the basis of the mask alignment scope measurement results, the position of the wafer relative to the mask is calculated, thereby aligning the wafer.
In the above arrangement, a means for aligning the mask will be described with reference to FIGS. 9A and 9B.
FIG. 9A shows the state of the apparatus in which only the mask 20 is arranged, and FIG. 9B shows the state of the apparatus in which the mask has aligned for the stage coordinate system at the rotational position. Four mask alignment marks 105a to 105d are formed on a membrane on the mask 20. The mask alignment marks 105a and 105b are used for measuring positions in the X direction, and the mask alignment marks 105c and 105d are used for measuring positions in the Y direction. The mask alignment scopes 11a, 11b, 11c, and 11d move to the respective mark positions. Every time each of the mask alignment marks is measured, the reference mark formed on the wafer stage moves directly under (not shown) each of the mask alignment marks to detect the position of the mask alignment mark 105 relative to the reference mark. The rotation and position (each of the positions in the X and Y directions) of the mask relative to the wafer coordinate system are measured on the basis of the measurement results of two points in the X direction and those in the Y direction. On the basis of the measurement results, the mask stage is driven to align the rotational position to the wafer coordinate system. Since the mask stage 41 can be driven only about the center of the mask as a rotation axis, the shift of the position (each of the positions in the X and Y directions) of the mask relative to the stage coordinate system cannot be corrected. However, the measurement result is reflected as a correction amount in aligning the wafer stage.
The dotted lines in FIG. 9B represent the positions of the mask alignment scopes 11a, 11b, 11c, and 11d before alignment. The mask alignment scopes are mounted on the respective 2-axis stages 501 in order to move to the respective mask alignment mark positions. At least three mask alignment scopes are required to calculate the mask rotational position, and at least three 2-axis stages are required as well.
However, in the above-described technique, the mask alignment marks, mask alignment scopes, and reference marks need be aligned in mask alignment. Also, along with driving the mask stage, the wafer stage (reference mark) and four (at least three) mask alignment scopes need to move in alignment. In the alignment measurement, an error of the measurement result may occur in accordance with the position of the mask relative to each of the mask alignment scopes. However, the mask alignment scope and mask stage are on different coordinate systems, and are moved by respective positional measurement systems. Thus, when the mask is aligned or the like, even if the mask alignment scope and mask stage move to the same position, the relative positions of the mask alignment mark and the mask alignment scope may be different. Therefore, the error of the alignment measurement result occurs, and the precision of the mask alignment is decreased.
Since the mask rotational position is detected by dividing the positional difference between the two separated marks by the distance, the distance between the two marks is preferably large. However, since the marks need to be arranged in the membrane to measure the alignment, a sufficient distance between the two marks cannot be ensured.
Also, in this arrangement, the mask alignment scope measures the mask position through the mask stage and a masking blade. Hence, the distance (working distance) between the scope and mask becomes large, and the mask alignment scope also becomes large. Further, the mask stage, masking blade, and stage for aligning the mask alignment scope are crowded around the exposure position, impairing maintenability of the apparatus.